JPS6017035B2 - Metal electrolytic refining method and its equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrolytic cell for producing metals by electrolysis of a molten electrolyte, and in particular to an electrolytic cell structure in which the molten electrolyte is denser than the target metal.

Although the invention is detailed below with respect to the production of magnesium from a molten electrolyte with a high content of magnesium chloride, the invention can also be applied to electrolyzers for carrying out other electrolytic processes in which similar problems arise. It is something. In producing magnesium from a relatively dense electrolyte, the cathode and electrodes of the electrolytic cell are arranged so that the surfaces of the electrolytic cell, which are arranged vertically or vertically, face each other and are substantially parallel to each other. has been done.

Water-like chlorine gas bubbles emerge from the surface of the anode and diverge slightly upwards, and a coating of magnesium forms on the surface of the cathode, which moves upward. Such an upwardly moving magnesium film is bridged at the top edge of the cathode and is deflected laterally away from the cell without contacting the evolving chlorine gas. If it comes into contact with chlorine, magnesium will undergo a reverse reaction.
Such molten magnesium is collected in a tapping well over the body of molten electrolyte and is removed from the collection well by a siphon discharge device in a substantially conventional manner. maintained at temperature.

It is clear that the electrolyte in the electrolytic cell should be maintained at a temperature above the melting point of the target product metal. It is already well known that the current efficiency of an electrolytic cell is greatly improved if the temperature of the electrolyte can be kept as low as possible (provided that the temperature is higher than the melting point of the product metal). It is.

As described in U.S. Pat. No. 3,396,094, the upward flow of molten product metal magnesium is collected in a sekisoku-style steel collection vessel contained substantially entirely in molten electrolyte in a tapping well. It has been found that when this is done, the temperature of the electrolyte can be maintained within 200° above the melting point of magnesium without any operational difficulties. The temperature of the electrolyte is controlled to a value as low as possible above the melting point of the product metal, but a somewhat higher electrolyte temperature is maintained at all times to avoid operational problems resulting from solidification of the product metal at the cathode. It is necessary to maintain the

Therefore, the heat generated in the electrolyte by resistive heating of the electrolyte should be somewhat greater than the normal amount of electrolyte heat loss, and temperature control of the electrolyte should be carried out by variable controlled cooling of the electrolyte. should be done. As well as for the metal collection function, the tapping well is also used for the introduction of molten electrolyte feedstock and is therefore a hinged well that allows for the introduction of molten chloride feedstock and electrolyte auxiliary components and the removal of molten metal. A sectional cover was provided.

Control of electrolyte temperature has been achieved by opening and closing such covers to achieve controlled air cooling of the electrolyte. In comparison to earlier devices such as the electrolyzer described in U.S. Pat. No. 2,785,121, U.S. Pat.
The operation of the electrolyzer described in '094 resulted in a significant improvement in current efficiency and a substantial reduction in the formation of solid sludge at the bottom of the electrolyzer.

The reason for this is that in early types of electrolyzers, the accompanying combustion of the molten magnesium was generally effected by the tapping device. In operation, the operational life of such an electrolyzer was approximately one year. After that period, the operating efficiency of the electrolytic cell deteriorated, and the electrolytic cell had to be shut down and repaired. In particular, it was necessary to remove sludge from the bottom of the electrolyzer. It was the accumulation of sludge, especially at the bottom of the tank, that led to the reduction in operating efficiency. The formation of such sludge is due in part to the formation of magnesium oxide due to the oxidation of some exposed metallic magnesium, and in part to the formation of MgC12 feedstock prior to or during introduction into the electrolyzer. This is due to the introduction of Mg0 and magnesium oxychloride into the molten MgC12 as a result of the hydrolysis of . The presence of fine solid particles in the electrolyte can contaminate the surface of the cathode with oxidized deposits that prevent the maintenance of a continuous metal film on the cathode surface, and Current efficiency will decrease until the deposits are removed.

A substantial portion of the sludge buildup is due to hydrolysis of the electrolyte as a result of contact with the atmosphere within the tapping well during electrolyte temperature control operations performed by lifting (or opening) the tapwell cover. This was acknowledged and not done.

According to the present invention, 'a} the molten metal is collected in the form of a top light layer to shield the electrolyte from atmospheric moisture, and 'b} the space above the molten metal is covered with a heat insulating cover. maintain an atmosphere in the space above the molten metal to reduce heat loss from the molten metal as low as possible and 'c' reduce oxidation of such metal to an insignificant level; {d) By passing a heat exchange fluid through the heat exchange means in direct contact with the molten electrolyte, the electrolyte temperature is reduced and maintained at the desired value, thereby significantly reducing sludge formation and extending the operating life of the electrolyzer. It has been found that moderate increases can be achieved while still retaining the benefits of controlling the electrolyte temperature close to the molten metal temperature to achieve high current efficiency.

The heat exchanger is most conveniently positioned to extend through the top of the product collection chamber, through the molten metal layer, and downward into the molten electrolyte.

The desired atmosphere above the molten metal is established by substantially sealing the space from the atmosphere and/or by introducing into the space an inert gas (e.g. argon) or an antioxidant gas (
For example, by blowing S02, SF6 or other antioxidant gases commonly used in magnesium casting operations. The addition of argon in such amounts as to maintain the atmospheric oxygen concentration in such spaces near or below 1% has been found to be effective in preventing rapid oxidation of molten magnesium metal at operating temperatures. did. Heat exchangers are used both to remove heat from the electrolyte by passing a relatively cold fluid through it, and to introduce heat into the electrolyte by using a hot fluid as the heat exchange medium that circulates in the heat exchanger. It can be made to perform the functions of As an alternative to using a heat exchanger as a means of introducing supplementary heat into the electrolytic cell, other heating schemes may be used to increase the temperature of the electrolyte in the tapping well. Such supplementary heat can therefore be supplied to the electrolyte by passing an alternating current between electrodes in contact with the electrolyte. As another alternative, supplementary heat can be introduced directly into the toplight metal layer,
In particular, it is possible to directly introduce supplementary heat to increase the fluidity of such metals before they are removed, for example such means are heat sinks or preferably immersion heaters, electricity or The heat source is a gas flame. Preferably, the heat exchanger described above, when used for cooling, absorbs only a substantially negligible amount of heat from the molten metal layer of the top light. A preferred form of the heat exchanger has an outer tube collar supported through the cover of the tapping well and extending downwardly through the molten metal layer and into the electrolyte.

A metal heat exchange tube having an outer diameter smaller than the inner diameter of the outer collar extends downwardly through the outer collar and is sealed during exchange at the lower end of the collar. In this way the heat exchange tubes are effectively insulated from the molten metal layer. It is preferable to fill the space between the outer tube collar and the heat exchange tube with a heat insulating material. The heat exchange tube extends below the outer tube collar and reaches a position near the bottom of the electrolyte in the tapping well. The lower end of the heat exchange tube is closed. Another tube of smaller diameter is provided coaxially with the heat exchange tube. This four-diameter tube serves as an exit port for the heat exchange fluid and is preferably formed from a refractory material to prevent backflow of heat from the heated exiting fluid. The advantage of this form of heat exchanger is that it can be removed for replacement without disturbing the cover of the tapwell. Alternatively, a simple U-shaped heat exchanger may be attached to two collars on the cover of the tapping well.

Although such a device is simple, it is somewhat difficult to replace because the cover of the tapping well must be removed. An example of an electrolytic cell for producing magnesium according to the present invention will be explained with reference to the accompanying drawings. As shown in Figure 1, the electrolytic cell has an outer shell 1 made of steel, a thermally insulating layer 2, and a material that can withstand both molten magnesium and molten chloride electrolyte (which may contain small amounts of fluoride). It has a thick refractory lining 3 consisting of.

This electrolytic cell includes a fireproof curtain worm 4, and a long hole 5 is formed in the curtain worm.

A curtain worm 4 separates a tapping well 6 from an electrolytic chamber 7 in which a series of parallel anodes 8 are disposed, held by an insulating cover 9, together with a series of parallel cathodes 10. MgC12 and other electrochemically more positive metal halides (e.g. NaC1
, KCI, and CaC12) and is denser than molten magnesium. During operation, chlorine is generated at the anode 8,
gathers in the top space of the electrolytic chamber 7 under slightly negative pressure,
From there it is discharged via a discharge port (not shown). A drop of molten magnesium forms on the surface of each cathode and drains from the electrolyte 7 towards the tapping well 6. For this purpose, each cathode 1 is provided with an inverted trough 11 sloping upwards to transport the formed metal melt from the electrolytic chamber 7 through the holes 5 in the wall 4 into the tapping well 6. (See US Pat. No. 3,396,094). The produced metal forms a top layer 12 on the molten electrolyte in the tapping well 6, and the layer! The lowest position of hole 2 is above the top of hole 5. The production metal layer 12 is defined below the top space 14 by a highly insulating fixed cover 15, which is sealed to the cage wall above the tapping well 16, as will be explained in more detail below. or more heat exchange units 17 are mounted on the cover 15, each unit being held by a steel collar 18 extending downwardly below the lower operating limit position of the metal product layer 12; Insulation layer (not shown)
It consists of a steel heat exchange tube 19 separated from its collar 18 by a refractory conduit 20 coaxial therewith.

In operation, lined air is blown into the upper end of tube 19 and exhausted via conduit 20. Only the portion of the tube 19 below the lower margin of the collar 18 performs a substantial heat exchanger function. Another embodiment of the heat exchanger is shown in FIG.

The exchanger has a U-shaped heat exchange tube 19' mounted at each end in a collar 18 retained in cover 15. A plurality of spaced apart steel electrodes 22 project through the walls of the electrolytic cell into the electrolyte region and apply an alternating current (
AC) is applied.

The cover 15 is adapted to form a substantially gas-tight seal with the refractory lining 3 of the electrolytic cell, as shown in FIG. For this purpose, a backing layer 24 of salt (e.g. NaCI) which remains solid at the operating temperature is applied to the refractory lining 3 of the electrolyzer wall and to the refractory lining 23 of the catabar 15.
A compressible rubber-like sealing material 2 placed between the
5 is placed between angle members 26 and 27 (forming part of the outer shell of the electrolytic cell and part of the cover 15, respectively). The sealant 25 is, for example, 2 sold by Parker Packing Company of Carson City, Nevada, USA.
Can be made from high temperature resistant silicone gasket material that can be used for long periods of time at temperatures up to 3. Sealing member 25 acts as a barrier against atmospheric ingress, while solid salt layer 24 acts as a thermal barrier to protect member 25. The sealing mechanism shown in FIG. 2 extends over three sides of the cover 15.

On the fourth side of the cover (in this case the side of the cover 15 facing the cover 9 in FIG. 1), the sealing layer of salt is continuous between the electrolyzer refractory lining 3 and the refractory lining 23 of the cover 15. However, a compressible silicone rubber gasket 25 is located between the vertical surfaces of cover 9 and cover 15. In operation, a slow flow of dry argon (or other inert gas such as nitrogen) is introduced into the head space 14 through a gas inlet 26 provided in the cover.

0.6 skin x 4 without using the aforementioned rubber sealant
．． For a five-skin-sized tapping well, an argon flow of 2/min can reduce and maintain the oxygen content of the gas in the headspace to about 1%. The top space 14 in the tapping well preferably varies between 10 and 20 skins in the vertical direction.

When the above-mentioned highly insulated fixed cover 15 is used, the metal layer 1
It was found that No. 2 remained substantially molten even when the temperature of the electrolyte in the tapping well dropped to less than 5° C. above the melting point of magnesium (65 loo). The reason for this is that the sum of heat loss due to conduction to the heat exchanger and electrolyzer walls, and heat loss due to fin radiation from the surface of the molten magnesium to the cover is substantially reduced. However, in practice, it is preferred to maintain the electrolyte temperature within the range of 660-670 qo as a preventive safeguard against sudden and unexpected outages and input interruptions.

Such an accident would cause the electrolyte temperature in the electrolytic cell to drop to approx.
It will descend at a rate of 0.00/hr. In operation, the electrolyte temperature is between 660 and 670, depending on the operation of the heat exchanger 17.
CO, and doing so provides good current efficiency. However, the electrolyte temperature is raised to about 68,030° C. for a period of time just before tapping the Ura metal out of the system to increase the fluidity of the metal and to prevent the possible formation of solidification.
(solidification) It is desirable to remelt the metal. After removing the metal from the system, the electrolyte temperature can be returned to the desired operating temperature of about 660-670° C. by operation of a heat exchanger. The electrolyte is heated using an AC. It can also be carried out by a resistance heating method.

Alternatively, a stream of highly heated gas may be blown into a heat exchanger to accomplish such a purpose. MgC1
2 (Electrolyte) Since the introduction of air through the raw material supply port should be minimized as much as possible, the molten MgC12 raw material is sealed in the cover 15 and penetrates the molten metal layer 12 into the molten electrolyte region below. It is supplied through a conduit 27 that extends up to 0.

The mouth of the conduit 27 is closed with a lightweight removable cover 28 to minimize the introduction of atmospheric air onto the exposed surfaces of the electrolyte. Similarly, metal is taken out of the system via a small conduit 29 provided in the cover 15.
This conduit is also provided with a lightweight removable cover 30.

Also around the entrance of the conduit 29 (not shown),
A solid salt seal is provided to cooperate with cover 30. This can also be supplemented with a rubber seal (such as 25 above) to reduce the amount of argon required to be introduced into the cell. A very important advantage of the device according to the invention is that the cooling operation of the heat exchanger 17 is under the control of a submerged satermostat inserted into the electrolyte and that the operation of the heat exchanger is stopped and the operation of the AC heating circuit is started. It is a scale that is performed automatically under the control of a timer controller.

At a suitable time prior to a given scheduled metal extraction operation, the temperature 0 set of the thermostat is raised to 68,000 to prepare the electrolyzer for metal extraction. In operation, the sludge volume in the electrolytic cell of the present invention is 20% per ton of product metal.
It has been found that it can be kept as low as k9 or lower.

In contrast, in the operation of the electrolytic cell described in U.S. Pat. No. 3,396,094, 6
0k9 sludge is produced. Furthermore, it was also found that the amount (rate) of depletion of the carbon anode was reduced to about 1/3 of the amount of depletion in the prior art.

As a result of these two factors, the operational service life of an electrolyzer between major electrolyzer refurbishments can extend from one year to two to three years or more. One of the other major advantages of the electrolytic cell of the present invention is that it does not require the large diameter of the previously used steel tubes. Periodic replacement of the heat exchange unit shown in FIG. 1 can be accomplished very easily with very little exposure of the electrolyte to the atmosphere. For this reason, the heat exchanger is attached to the cover 15 by means of a bolted flange connection with a gas-tight heat-resistant gasket 31.

[Brief explanation of drawings]

The attached drawings show an example of an electrolytic cell according to the present invention. FIG. 1 is a longitudinal sectional view of the electrolytic cell, FIG. 2 is a partially enlarged sectional view of the cover of the tapping well, and FIG. 3 is a plane perpendicular to FIG. 1. FIG. 3 is a longitudinal cross-sectional view in which a U-shaped heat exchanger is provided. 1: Tank outer shell, 2: Heat insulating layer, 3: Heat resistant lining, 6: Tapping well (product collection chamber), 7: Electrolyte, 8: Anode, 10: Cathode, i2: Molten metal layer, 14: Neck space, 15: Cover, 17: Heat exchanger. RiG. ' 〃G. 2 stroke G. 3

Claims (1)

[Scope of Claims] 1. A method for producing a metal by electrolysis of a molten electrolyte having a higher density than the target metal, comprising: at least one vertically disposed anode immersed in the molten electrolyte in an electrolytic chamber; a vertically disposed cathode to electrolyze the molten electrolyte and transfer the resulting metal to a product collection chamber, comprising: (a) a supernatant layer to shield the electrolyte from atmospheric moisture; (b) maintaining an insulating cover over the molten metal to reduce as much as possible heat loss from the molten metal; (c) collecting the molten metal in a product collection chamber; (d) the temperature of the molten electrolyte is controlled by a heat exchange fluid in a heat exchanger in direct contact with the molten electrolyte; A method for manufacturing the above-mentioned metal, characterized in that the metal is lowered to a desired level and maintained by passing the metal through the metal. 2. A method according to claim 1, characterized in that the removal of heat from the molten electrolyte is carried out without significantly removing heat from the supernatant molten metal layer. 3. A method according to claim 1, characterized in that a flow of inert gas is introduced into the space above the molten metal layer to maintain a non-oxidizing atmosphere therein. 4. A method according to any one of claims 1 to 3 for producing metallic magnesium by electrolysis of magnesium chloride, wherein molten electrolyte is
A method characterized in that the temperature is maintained in the range from 6 to 670°C. 5. An apparatus for producing metal by electrolysis of a molten salt having a higher density than the target product metal, which comprises: an electrolytic chamber; a product metal collection chamber communicating with the electrolytic chamber; arranged vertically in a row;
Interleaved anode and cathode electrodes; means provided on each cathode for transporting the metal produced at the cathode into the product metal collection chamber; top surface during normal operation of the electrolyte in the product metal collection chamber; The top space of the product metal collection chamber is isolated from the top space of the electrolytic chamber by a curtain wall that extends downward to a position below the top of the product metal collection chamber; and the fixed cover includes an electrolyte charging conduit extending downwardly below the normal operating top surface of the molten electrolyte, a product metal outlet, a removable lid for the product metal outlet, and The above-mentioned device is characterized in that it includes a heat exchanger extending downwardly below the top surface of the molten electrolyte during normal operation to exchange heat in contact with the molten electrolyte. 6. A patent in which the heat exchanger is provided with insulation means above the upper surface of the molten electrolyte during normal operation to prevent significant absorption of heat from the molten metal collected in the product metal collection chamber. Apparatus according to claim 5. 7. The apparatus of claim 6 including a plurality of spaced apart electrodes in the lower portion of the product metal collection chamber for imparting electrical resistance heating to the molten electrolyte in the product metal collection chamber. 8. The heat exchanger has a straight flow tube extending downwardly and closed at the lower end for the heat exchange fluid medium, and a heat exchange fluid medium return tube arranged in the flow tube. Apparatus according to paragraph 5, 6 or 7. 9 The flow tube is held spaced apart from the tubular support surrounding it and is connected to the tubular support below the normal operating surface of the molten electrolyte, and 9. The device of claim 8, wherein the body is supported in and removable from the fixed cover. 10. The device according to any one of claims 5 to 9, wherein a gas inlet is provided in the fixed cover for introducing an inert gas into the top space of the product collection chamber.